All the DIY planars that I have seen were completely built from scratch. I'm not aware of kits of any real size. That's probably because it would be super easy to violate a Magnepan patent, and Magnepan takes such matters seriously. The Magnepan vs Apogee case was a good example of that.
Anyway, there is a seller in Birmingham who can supply almost any magnet you can imagine. That would be the hardest of the parts acquisition, wouldn't it?
Quite a few people are buying Magnepan speakers on the used market, and seriously modifying them with stellar results. There's no law that says you can't combine multiple old pairs into a new state of the art beast. Of course, they'll sound like crap in the end unless you move up to vinyl.